3 Sums and Integrals

3

Sums and Integrals

Definite integrals are limits of sums. We will therefore begin our study of integrals

by reviewing finite sums and the relation between sums and integrals. This will allow

you to understand approximate values of integrals even when you can��t evaluate the

integral analytically (another instance of gaining number sense!). The first topic,

finite sums, is very elementary but I don��t know any good references so I��m including

a reasonably complete treatment.

3.1

Finite sums

The preparatory homework for this sections deals with the nuts and bolts of writing

19

?

3

finite sums. If given a sum such as

you should easily be able to tell what

n

?

2

n=5

explicit sum it represents: how many terms, what are the first few and the last, how

would you write it using an equation with . . . and so forth. The above sum, for

3 3

3

example, contains 15 terms and could be written as + + �� �� �� + .

3 4

17

It is a little harder going the other way, writing a sum in Sigma notation when you

are given its terms. One reason is that there is more than one way to do this. For

example there is no reason why the index in the previous sum should go from 5 to 19.

There have to be fifteen terms but why not write it with the index going from 1 to 15?

Then it would look like

15

?

3

.

n+2

n=1

Another natural choice is to let the index run from 0 to 14:

14

?

n=0

3

.

n+3

All three of these formulas represent the exact same sum.

Another di?culty is that you need to know tricks to represent certain patterns with

formulas. Really this is not a di?culty with smmations as much as with writing

a formula to represent the general term an of a given sequence. Realize that these

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problems are inherently the same: writing the nth term of a sequence as a function of n

and writing the summand in a summation as a function of its index. The preparatory

homework starts o? with sequence writing and then has you do some summations as

well.

Here are some tricks to write certain patterns. The term (?1)n bounces back and

forth between +1 and ?1, starting with ?1 when n = 1 (or starting with +1 if your

sum has a term for n = 0). You can incorporate this in a sum as a multiplicative

factor and it will change the sign of every second term. Thus for example, to write

the sum 1 ? 2 + 3 ? 4 + �� �� �� ? 100 you can write

100

?

n=1

(?1)n+1 �� n .

Note that we used (?1)n+1 rather than (?1)n so as to start o? with a positive rather

than a negative term.

When the sum has a pattern that takes a couple of steps to repeat, the greatest integer

function can be useful. For example, 1 + 1 + 1 + 2 + 2 + 2 + 3 + 3 + 3 + �� �� �� + 10 + 10 + 10

?

30 ?

?

n+2

can be written as

.

3

n=1

Sequences and sums can use definitions by cases just the way functions do. Suppose

you want to define a sequence with an opposite sign on every third term, such as

?1, ?1, 1, ?1 ? 1, 1, . . .. You can do this by cases as follows.

?

?1

n is not a multiple of 3

an =

1

n is a multiple of 3

Although you will not be required to know this, you can use sophisticated tricks to

avoid this kind of definition by cases. One way1 is to use the greatest integer function:

an = (?1)?2(n?1)/3? .

Notational observations: A sequence denoted a1 , a2 , a3 , . . . could just as easily be

written as a function a(1), a(2), a(3), . . .. The value of a term an is a function of

the index n and there is no di?erence whether we write n as a subscript or as an

argument.

1

Another way is to use complex numbers, but you��ll have to ask me about that separately if

you��re curious.

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Series you can explicitly sum

We will learn to sum three kinds of series: arithmetic (accent on the third syllable)

series, geometric series and telescoping series.

Arithmetic series

An arithmetic series is a sum in which the terms increase or decrease by the same

amount (additively) each time. You can always write these in the form an = A + dn

where A is the initial term and d is how much each term increases over the one before

(it could be negative if the terms decrease). Here you should start the sum at n = 0

or else use the term A + (d ? 1)n. The standard trick for summing these is to pair

up the first and last, the second and second-to-last, and so on, recognizing that each

pair sums to twice the average and therefore that the sum is the number of terms

times the average term. Here is an example in a particular case and then the general

formula.

Example: Evaluate

29

?

n. There are 17 terms and the average is 21, which can be

n=13

computed by averaging the first and last terms: (13 + 29)/2 = 21. Therefore, the

sum is equal to 17 �� 21 = 357.

General case: Evaluate

M

?

A + dn. There are M + 1 terms and the average is A +

n=0

(dM/2). Therefore the sum is equal to (M + 1)(A + (dM/2)) = A(M + 1) + dM (M + 1)/2.

Geometric series

A geometric series is a sum in which the terms increase or decrease by the same

multiplicative factor each time. You can always write these in the form an = A �� rn

where A is the initial term and r is the factor by which the term increases each time.

If the terms decrease then r will be less than 1. If they alternate in sign, r will be

negative. Also, again, A will be the initial term only if one starts with the n = 0

term or changes the summand to A �� rn?1 .

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The standard trick for summing these is to notice that the sum and r times the sum

are very similar. I��ll explain with an example.

Example: Evaluate

10

?

n=1

7 �� 4n?1 .

To do this we let S denote the value of the sum. We then evaluate S ? 4S (because

r = 4). I have written this out so you can see the cancellation better.

S ? 4S = 7 + 28 + 112 + �� �� �� + 7 �� 49

? (28 + 112 + �� �� �� + 7 �� 49 + 7 �� 410 )

= 7 ? 7 �� 410 .

From this we easily get S = (7 ? 7 �� 410 )/(1 ? 4) = 7(410 ? 1)/3.

General case: Evaluate

M

?

n=1

A �� rn?1 .

Letting S denote the sum we have S ? rS = A ? Arn and therefore

S=A

1 ? rn

.

1?r

Infinite series

No discussion of finite series would be complete without a mention of infinite series.

There is a whole theory of convergence of infinite series that they teach in Math 104.

Here we��ll stick to what��s practical. It should be obvious that 1 + 2 + 4 + �� �� �� does

NOT converge, while 1/2+1/4+1/8+�� �� �� DOES converge, and in fact converges to 1.

There are eleven theorems and tests in the book about when series converge. From a

practical point of view, all you need is two things: the definition, and an example.

?

Definition: An infinite sum ��

n=1 an is said to converge if and only if the

?M

partial sums SM = n=1 an form a convergent

?sequence. In other words,

if limM ���� SM exists and is equal to L, then ��

n=1 an is said to equal L.

n

M

Example:

?�� If ann = (1/2) then SM = 1 ? (1/2) . Clearly limM ���� SM = 1 so we say

that n=1 (1/2) = 1.

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3.2

Riemann sums

In this unit we recap how areas lead to integrals and then, by the Fundamental

Theorem of Calculus, to anti-derivatives.

Areas under graphs

Thankfully, Sections 5.1�C5.3 do a nice job in explaining areas of regions under graphs

as limits of areas of regions composed of rectangles. I will just point out the highlights.

This figure shows a classical rectangular approximation to the region under a graph

y = f (x) between the x values of 2 and 6. The rectangular approximation is composed

of 16 rectangles of equal width, all of which have their base on the x-axis and their top

edge intersecting the graph y = f (x). The rectangular approximation is clearly very

near to the actual region, therefore the area of the region will be well approximated

by the area of the rectangular approximation. This is easy to compute: just sum the

width times height. The sum that gives this area is known as a Riemann sum.

Because the height is not constant over the little interval, there is no one correct

height. You could certainly cover the targeted area with your rectangles by always

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